When you close your AWS account, all resources on your account are marked for deletion, and on-demand billing for your resources stops. There are two reasons you might receive a bill after your account is closed:

If you incurred charges in the month before you closed your account, you'll receive a final bill for the usage incurred between the beginning of the month and the date you closed your account.

If you have any active capacity reservations on your account (EC2 Reserved Instances, RDS Reserved Instances, Redshift Reserved Instances, or ElastiCache Reserved Cache Nodes), you'll continue to receive a bill for these until the reservation period expires.